The present invention is directed to a method for inhibiting or preventing fluttering of a paper web passing through the drying section of a paper machine, within the area of a twin-wire draw.
The present invention is also directed to apparatus for carrying out the method, the apparatus including blow or nozzle boxes placed in the pockets formed by the wire-guide rolls and the wires themselves, and extending substantially over the entire transverse width of the web.
Running speeds of paper machine have been constantly increasing in recent years, with speeds being approached of 1500. The fluttering of the web therefore becomes a serious problem, hampering the running quality of the paper machine. The transfer of the web from the press section to the drying section, and the support of the web in an area of a single-wire draw, can be controlled with certain previously-known methods and apparatus. However, in the area of the twin-wire draw, in particular in the third and fourth operating groups of cylinders within a drying section, difficulties have been encountered at high running speed.
As used herein, a single-wire draw is a mode of passing the web over the heated drying cylinders, in which the web runs from one line of cylinders to the other supported by a drying wire, so that the web is between the drying wire and the cylinder surface on one line of cylinders, and on the other line of cylinders, the web is outside and the drying wire is situated between the cylinder surface and the web, with the web being supported by the drying wire in the draws running between the line of cylinders. An advantage of a single-wire draw is that the web is always supported by the drying wire, and there are no open draws at all, or at least no substantially long open draws, which reduces the risk of wrinkles and breaks in the web.
As used herein, a twin-wire draw is a prior-art mode of supporting and passing the web in conjunction with heated drying cylinders, in which an upper wire is used in conjunction with the upper cylinders and a lower wire is used in conjunction with the lower cylinders. These wires are guided by the surfaces of the drying cylinders and by the guide rolls placed between the drying cylinders, so that, on the upper line of cylinders, the web is pressed by the upper wire into direct drying contact with the surfaces of the upper cylinders, and, correspondingly, into drying contact with the surfaces of the lower cylinders, by the lower wire.
In the twin-wire draw, the web has generally had substantially long open draws when running from one line of cylinders to the other. These open draws have been subject to fluttering, resulting in breaks and wrinkles in the web. This drawback has been accentuated in the initial portion of the drying section, where the web is still relatively moist and therefore of low strength, with elastic properties conducive to fluttering.
Attempts have been made to eliminate this drawback by shortening the open draws of the web in the inital portion of the drying section, by placing the imaginary planes passing through the axis of the upper and lower line of cylinders at a distance from one another that is shorter than customary, or shorter than what would be optimal, e.g., in view of the efficiency of drying.
The possibility of providing the third and fourth drying groups of cylinders with a single-wire draw, has also been considered. However, this has been an exigent solution, because it results in a lowered evaporation efficiency and makes the arrangement of air conditioning more difficult.
Attempts have been made to reduce the fluttering of the paper-web in a drying section provided with a twin-wire draw, by shifting the felt guide rolls so that the paper-web runs a shorter distance without support. In U.S. Pat. No. 3,753,298, such a drying section is described. According to the paper "Engineering Consideration for Lightweight Paper Drying in High Speed Machines" (Paper Technology and Industry, July/August, 1978), the positioning of the rolls in accordance with U.S. Pat. No. 3,753,298 has been used in a Swedish paper machine, with which a speed of 853 m/min has been attained. However, fluttering of the web has continued to be a difficulty.
The fluttering of the paper-web has been discussed in the publication "Manufacture of Paper", Textbook and Manual of the Finnish Paper Engineers' Association III, Volume I, pages 699-700, where it is stated that fluttering of the edge of the web is generally not caused by currents of air, which has been a common belief. Under the circumstances, the fluttering of the web cannot be significantly prevented with guiding of air currents in the drying section, which has, however, been frequently attempted.